Debugging Optimized Code

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Debugging Optimized Code

• Steven Osman

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Debugging Optimized Code is Hard

• Un-optimized code has a correlation between
  source and object code
• Optimizing compilers rearrange, add and remove
  code. This results in
• Code location problems
• Data value problems

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Talk Overview

• Understanding the problems
• Introducing Source-Level Debugging of Scalar
  Optimized Code
• Classifying data-value problems
• Identifying data-value problems
• Whats next?

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The Code Location Problem
Source S1 y0 S2 abc S3 x2 S4 yz3 Optimized Assembly I1 ld r1, b ltS2gt I2 ld r2, c ltS2gt I3 ld r5, z ltS4gt I4 mul r6, r5, 3 ltS4gt I5 mov r4, 2 ltS3gt I6 add r3, r1, r2 ltS2gt
We want a breakpoint at S2
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The Data-Value Problem
Source S1 y0 S2 abc S3 x2 S4 yz3 Optimized Assembly I1 ld r1, b ltS2gt I2 ld r2, c ltS2gt I3 ld r5, z ltS4gt I4 mul r6, r5, 3 ltS4gt I5 mov r4, 2 ltS3gt I6 add r3, r1, r2 ltS2gt
What is the value of y at S2?
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Source-Level Debugging of Scalar Optimized Code

• By Ali-Reza Adl-Tabatabai and Thomas Gross
• Identifies data-value problems
• No changes to the optimized code
• Uses knowledge of optimizations

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Some Simple Terminology

• Expected value value a variable should have
• Actual value value stored in register
• Current Actual value IS expected value (true
  for all in unoptimzed code)
• Endangered Not current
• Noncurrent Actual IS NOT expected
• Suspect Actual may be expected

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Code Hoisting
E0 xu-v
E1 xyz
Bkpt1
Bkpt2 E2 xyz Bkpt3
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Code Hoisting
E0 xu-v
E1 xyz
E3 xyz Bkpt1
Bkpt2 E2 xyz Bkpt3
E2RedCopy(E3)
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Hoist Reaches
E0 xu-v
E1 xyz
E3 xyz Bkpt1
Bkpt2 E2 xyz Bkpt3
E2RedCopy(E3)
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Dead Code Elimination
E0 xyz Bkpt1
Bkpt3 E1 x Bkpt4
Bkpt2
Bkpt5 E2 xu-v Bkpt6
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Dead Code Elimination
E0 xyz Bkpt1
Bkpt3 E3 xyz E1 x Bkpt4
Bkpt2
Bkpt5 E2 xu-v Bkpt6
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Dead Reaching
E0 xyz Bkpt1
Bkpt3 E3 xyz E1 x Bkpt4
Bkpt2
Bkpt5 E2 xu-v Bkpt6
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Whats Next?

• Good paper, but it only warns programmer
• Suggestions about how to get the expected value
• Alok Ladsariyas talk